Summary
During the cruise, which is part of the Swedish pelagic monitoring program, the Skagerrak, Kattegat, the Sound, the Baltic Proper and the Gulf of Bothnia were visited. In the Gulf of Bothnia, SMHI and Umeå Marine Research Center (UMF) carried out a joint mapping of nutrients.
The cooling of the surface water had now gained momentum in all investigated areas. But temperatures above normal were noted in the Baltic Proper and the Bothnian Sea, while the temperature in the Skagerrak and Kattegat were partly lower than normal. The salinity in the surface water was normal in the Skagerrak, the Kattegat and the Gulf of Bothnia, while levels higher than normal were noted in parts of the Baltic Proper and the Bothnian Sea.
In the Skagerrak and Kattegat, the concentration of nutrients had increased markedly in the surface water since November, but the levels of all nutrients were within the normal range for the month.
In the Baltic Proper, the concentration of nutrients had increased in the surface layer since November, above all there had been a strong increase in inorganic nitrogen. In both the Eastern and Western Gotland basins, higher values of inorganic nitrogen and silicate than normal were measured below 100 meters depth. At station BCSIII-10 the concentration of phosphate and inorganic nitrogen was instead below normal and the entire water column was oxygenated. In the Bornholm Basin, the Hanö Bight and in the Arkona Basin, the concentrations of nutrients were mostly normal. In the Bothnian Sea, concentrations were slightly lower than normal in the eastern parts and at some of the stations in the northern parts. In the deep water, the levels were mostly normal, but in the central parts, levels were found high above normal. The phosphate levels in the surface water were lower than normal in the eastern parts and at some stations in the northern parts of the area. The phosphate content increased with depth and the levels in the deep water were much above normal at some stations. Silicate levels were generally higher in the surface water in the western and central parts of the area compared to the eastern. Here, too, the levels increased with depth and at many stations levels above normal were noted in the deep water.
In the Bothnian Bay, levels of inorganic nitrogen were below normal. In the deep water, the nitrogen content increased and showed great variation, normal levels and both lower and higher levels than normal. The phosphate content in the surface water was low, which is normal for the time of year. The levels increased with depth and in the deep water higher levels than normal were found. Silicate levels above normal were noted in the surface and in the deep water the levels increased further and here too the levels were higher than normal.
In the central basins of the Baltic Proper, hydrogen sulphide was measured from 70-80 m and in the remaining basins no hydrogen sulphide was measured. But in Hanö Bight and the Bornholm Basin, a lack of oxygen was noted from a depth of 70 meters. The lowest oxygen concentration was found at Hanö Bight and BY4 in the Bornholm Basin with 0.2-0.3 ml/l at 70 m. The oxygen content in the bottom water varied between about 5 ml/l in the Kattegat and 3 ml/l in the Sound, which is relatively low but just within the normal range. In the open Skagerrak, the oxygen situation was good and at the coastal station Släggö the concentration had increased from 3 ml/l in November to just over 5 ml/l.
No lack of oxygen was noted in the Gulf of Bothnia. But low levels, close to oxygen deficiency, were noted at the coastal station Gavik-1. Levels just above 4 ml/l were also noted in the southwestern part of the Bothnian Sea.